This invention relates to the manufacturing of acoustic resonators, and, more particularly, to the manufacturing of resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that has the potential for meeting these needs is constructed from acoustic resonators. These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of piezoelectric material is sandwiched between two metal electrodes. The sandwich structure is suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes via an impressed voltage, the piezoelectric material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At the mechanical resonance, the device appears to be an electronic resonator; hence, the device can act as a filter. The mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the device for a given phase velocity of sound in the material. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions less than 100 xcexcm in diameter and few xcexcm in thickness.
Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators and Filters (SBARs) include a thin sputtered piezoelectric film having a thickness on the order of one to two xcexcm. Electrodes on top and bottom sandwich the piezoelectric acting as electrical leads to provide an electric field through the piezoelectric. The piezoelectric, in turn, converts a fraction of the electric field into a mechanical field. A time varying xe2x80x9cstress/strainxe2x80x9d field will form in response to a time-varying applied electric field.
To act as a resonator, the sandwiched piezoelectric film is suspended in air to provide the air/crystal interface that traps the sound waves within the film. The device is normally fabricated on the surface of a substrate by depositing a bottom electrode, the piezoelectric layer, and then the top electrode. Hence, an air/crystal interface is already present on the topside of the device. A second air/crystal interface must be provided on the bottom side of the device. There are several approaches for obtaining this second air/crystal interface. Some of these approaches are described in U.S. Pat. No. 6,060,818 to Ruby et al., incorporated herein by reference in its entirety.
It is possible to manufacture a plurality of FBARs on a single substrate, such as a four inch diameter single crystal silicon wafer. The substrate is then diced to separate the multiple FBARs manufactured thereon. However, the process of sawing the substrate can damage the extremely thin FBAR resonators, so care must be taken during the dicing step.
There is a need for an improved process for manufacturing acoustical resonator structures. In particular, there is a need for an improved batch processing method for producing FBARs and SBARs.
In accordance with the invention, a method for batch processing acoustic resonators includes: depositing a first electrode on a top surface of a substrate; depositing a layer of piezoelectric material on said first electrode; depositing a second electrode on said layer of piezoelectric material; and removing material from a bottom surface of said substrate to reduce the thickness of the substrate and to reduce an electromagnetic influence in a resulting filter.
In accordance with another embodiment of the present invention, an acoustic filter is provided. The acoustic filter includes: a die substrate having a cavity formed on an upper surface thereof, said die substrate having a thickness of less than 19 mils; a plurality of resonator membranes formed on said die substrate. Each of said plurality of resonator membranes comprises a first electrode provided over said cavity on said upper surface of said die substrate; a piezoelectric material provided over said first electrode; and a second electrode provided over said piezoelectric material. A plurality of interconnects are provided on said die substrate providing electrical connections between said plurality of resonator membranes. A package is provided, the package including a die cavity formed on an upper surface thereof, said die substrate being mounted in said die cavity such that a primary current flowing along an upper surface of said die substrate creates a primary current magnetic field and an image current flowing along a ground plane beneath said die substrate creates an image current magnetic field, said primary current magnetic field and said image current magnetic field having opposite polarities.